BCY #23 and #24 are both good choices for D loop material. BCY #3 serving string is popular for tying peep sights. There are a lot of different types out there. The Archery Talk Forum has a lot of discussion on d loop and serving material.
For loops just get some standard BCY 24. For tying peeps and center serving I prefer BCY 62X in .018. For tying knock sets I use BCY 3D in .016. For rest cord you can use BCY 23 or D Braid. I like the smaller diameter for rests.
I like BCY D Braid for loop material. Thinner and more flexible gives me good results, just make sure you get a good burn on the end and the knot seated well because its a little trickier than the 23 & 24 loop material. For serving I use BCY 3D for most things, nock points, tying in peeps etc.
Both products are popular in the target archery world and i like them so much that I have carried them over to my hunting set ups as well.